1. Field of the Invention
The present invention relates to a fixing device, in an image information recording device such as a copying machine, printer and facsimile, for heat-fixing an unfixed toner image onto a surface of a recording medium such as a recording sheet, and more particularly to a fixing device of a belt-nip system using a belt for forming a nip section for fixation. Further, the present invention relates to an image forming device including this fixing device.
2. Description of the Related Art
A so-called heat-pressure roller fixing device has widely been used as a conventional fixing device wherein an unfixed toner image is passed through a press-contact region between a pair of heated rollers to thereby fix the unfixed toner image (hereinafter referred to as “roll-nip system”). FIG. 5 shows a cross-sectional view of a roll-nip type fixing device. In FIG. 5, numeral 101 denotes a fixing roller rotating in a direction shown by an arrow F, while 106 denotes a pressure roller rotating in a direction shown by an arrow G with the fixing roller 101.
The fixing roller 101 is formed to have a silicon rubber coated as an elastic layer 104 on a surface of a hollow layer 105 made of a metal having high heat conductivity such as aluminum, and further have coated thereon a coating layer 103 made of Teflon (R) having releasability. A halogen lamp 102 is arranged as a heat source in the hollow roller 105. The halogen lamp 102 is on/off-controlled by a temperature controlling circuit not shown based upon a signal from a temperature sensor 108 disposed on the surface of the fixing roller 101, thereby adjusting the surface of the fixing roller 101 to have a desired constant temperature.
On the other hand, the pressure roller 106 is formed to have a relatively thick heat-resistant elastic layer 110 such as a silicon rubber coated on a mandrel roller 109. The elastic deformation of the heat-resistant elastic layer 110 forms a press-contact section (nip section) N′ of the fixing roller 101 and the pressure roller 106.
A sheet P having an unfixed toner image T formed thereon advances in the direction shown by an arrow H to be inserted into the nip section N′, whereby the unfixed toner image T is fixed by an action of pressure and thermal energy. Since the sheet P passing through the nip section N′ is liable to wrap around the fixing roller 101 due to the adherence of toner, a separation pawl 107 is disposed for separating the sheet P. Disposing the separation pawl 107 allows to separate the sheet P subject to the fixing operation from the fixing roller 101, whereby the sheet P is discharged from the device.
However, in the case where a high-speed fixing is intended by use of the above mentioned fixing system, it is necessary to give thermal energy and pressure sufficient to perform the fixing operation to the unfixed toner image T and the sheet P. This needs to widen the nip section N′ (nip width) in proportion to the fixing speed. In order to widen the nip section, there are various methods including a method for increasing a load between the rollers 101 and 106, a method for increasing a thickness of the elastic layer 104 or a thickness of the heat-resistant elastic layer 110, a method for increasing the diameters of both rollers 101 and 106, and the like.
The method for increasing the load between both rollers or the method for increasing the thickness of the elastic layer entails non-uniformness in the shape of the nip width along the shaft of the roller that is caused by a bending of the roller, non-uniform fixing or wrinkle on the sheet, thus having a limitation. Further, the method for increasing the diameter of the roller has no problem in quality as described above, but entails a problem that the device is large-sized and that the time for rising the temperature of the fixing roller 101 from room temperature to a fixable temperature (generally referred to as “warm-up time”) is prolonged.
In order to solve these problems to cope with a more high-speed operation, a system using a belt has been proposed as shown in FIG. 2 in the Patent Reference 1 (the system using a belt for forming the nip section is referred to as “belt-nip system” hereinafter). FIG. 6 shows a cross-sectional view of a fixing device of such a belt-nip system.
The fixing device shown in FIG. 6 has a main section formed of a fixing roller 111 rotating in a direction shown by an arrow J and a pressure belt module 116. The pressure belt module 116 has a main section formed of an endless belt 131 looped and stretched around three rollers of an inlet roller 128, pressure roller 129 and a tension roller 130 and a pressure pad (pressure member) 127 that is pressed against the fixing roller 111 via the endless belt 131.
The endless belt 131 rotates in a direction shown by an arrow K and forms a nip section N″ by being in contact with the fixing roller 111 such that the area between the inlet roller 128 and the pressure roller 129 wraps around the fixing roller 111. The pressure pad 127 is arranged in the endless belt 131 so as to be pressed against the fixing roller 111 via the endless belt 131. The pressure roller 128 is urged toward the surface of the fixing roller 111 via the endless belt 131 at the most downstream side with respect to the nip section N″ in the rotating direction (in a direction shown by an arrow K) of the endless belt 131, thereby forming a terminal end of the nip section.
Like the fixing roller 101 in the fixing device of a roll-nip system shown in FIG. 5, the fixing roller 111 is formed to have a silicon rubber coated as an elastic layer 114 on a surface of a hollow layer 115, and further have coated thereon a coating layer 113. A halogen lamp 112 is arranged as a heat source in the hollow roller 115.
In the fixing device of a belt-nip system shown in FIG. 6, a sheet 127 having an unfixed toner image 126 formed thereon is inserted into the nip section, whereby the unfixed toner image 126 is fixed by pressure and thermal energy at the nip section. This configuration can easily widen the nip width between the endless belt 131 and the fixing roller 111 compared to the nip width in a conventional roll-nip system (can form a wide nip), whereby it is possible to cope with a high-speed operation. Further, this configuration can attain a small-sized device than the fixing device of a roll-nip system when compared to each other with the same fixing speed.
However, there is a limitation to cope with the high-speed operation even in the fixing device of such a belt-nip system. Although the belt-nip fixing device can form a wide nip, there may be a case where, when a great number of sheets are intended to be subject to a fixing operation during a short period with high speed, the surface temperature of the fixing roller 111 temporarily reduces to thereby cause defective fixing when a number of copied sheets exceeds some numbers. This phenomenon is referred to as a temperature droop. This is a phenomenon caused by the following. Specifically, the silicon rubber coated as the elastic layer on the core of the fixing roller 11 serves as a thermal resistance, so that the heat is not rapidly transmitted to the surface of the fixing roller even if sufficient heat quantity is given to the inside of the roller. The temperature droop becomes great particularly when a thick paper having a great heat capacity is used. When the temperature droop occurs, it is impossible to give sufficient heat quantity to the sheet even if the wide nip is formed, whereby the merit of the belt-nip system cannot sufficiently be utilized.
A system for simultaneously using an external heating has been proposed (see the Patent Reference 2, for example) in order to improve this temperature droop. FIG. 7 shows a cross-sectional view of a fixing device of a belt-nip system and using an external heating. Although the fixing device shown in FIG. 7 is basically the same as the fixing device shown in FIG. 6, it has an external heating roller 132 mounted thereto that comes in contact with the surface of the fixing roller 111 for heating the fixing roller 111 not only from its inside but also from its outside by utilizing a heat source arranged in the external heating roller 132. However, since the contact width of the external heating roller 132 and the fixing roller 11 is narrow, the thermal energy of the external heating roller 132 cannot sufficiently be given to the fixing roller 111, with the result that it is difficult to obtain an effect as expected by simultaneously using the external heating. Further, heat radiation from the external heating roller 132 becomes great, so that it is desired to be improved from the viewpoint of efficiency of use of energy.
As the belt-nip system, not an embodiment wherein the belt at the pressure side is stretched as explained above, but a so-called free-belt-nip system has been proposed (see the Patent Reference 3, for example) wherein the belt is free and comes in contact with the fixing roller at the fixing side with the belt being rotatable with the fixing roller. This configuration has a merit of the belt-nip system and also entails the problem of the temperature droop involved with the high-speed operation and the problem of the efficiency of use of energy as described above, like the embodiment wherein the belt at the pressure side is stretched as explained above. Accordingly, the “belt-nip system” means here a concept including the “free-belt-nip system”.
[Patent Reference 1]
Japanese Published Unexamined Patent Application No. Hei 5-150679
[Patent Reference 2]
Japanese Published Unexamined Patent Application No. Hei 11-7216
[Patent Reference 3]
Japanese Published Unexamined Patent Application No. Hei 8-262903